Sub power plane to provide EMC filtering for VLSI devices

Abstract

Electromagnetic filtering for a VLSI device having multiple power input leads is realized by employing a sub-power plane which is physically separate from a main power distribution system on a circuit board. The sub-power plane is placed directly under a corresponding VLSI device. Decoupling capacitors are connected to the sub-power plane and, in turn, to each of the power input leads on the VLSI device. Power is supplied from the main power distribution system to the sub-power plane via a ferrite bead type filter.

I claim: 
     
       1. A circuit board having a prescribed surface comprising: a ground system;   a main power distribution system being disposed in a prescribed planar relationship to the prescribed surface of the circuit board; and   at least one sub-power plane disposed in prescribed spatial relationship with a physically separate from said main power distribution system, said subpower plane being in the same planar relationship as said main power distribution system to the prescribed surface of the circuit board and being adapted to be connected to at least one VLSI device having multiple power input leads.   
     
     
       2. A circuit board as defined in claim 1 wherein said at least one sub-power plane is adapted to be connected directly under at least one corresponding VLSI device. 
     
     
       3. A circuit board as defined in claim 2 further including a plurality of capacitors and wherein said at least one sub-power plane is adapted to connect a first capacitor to a first power input lead of said VLSI device and to connect at least a second capacitor to a second power input lead of said VLSI device. 
     
     
       4. A circuit as defined in claim 3 and wherein said sub-power plane is adapted to connect said plurality of capacitors to said multiple power input leads of said VLSI device on a one-to-one basis. 
     
     
       5. A circuit board as defined in claim 3 further including a filter connecting said sub-power plane to said main power distribution system for supplying power to said sub-power plane. 
     
     
       6. A circuit board as defined in claim 5 wherein said filter comprises a capacitor, a ferrite bead and one of said plurality capacitors and is arranged in a π configuration. 
     
     
       7. A circuit board as defined in claim 6 wherein said ground system is a ground grid and wherein each of said capacitors has a terminal adapted to be connected to said ground grid. 
     
     
       8. A circuit board as define din claim 3 wherein said main power distribution system is a main power plane having said at least one sub-power plane disposed therein and physically separated therefrom, said sub-power plane being spatially disposed in the same plane as said main power plane relative to the prescribed surface of the circuit board. 
     
     
       9. A circuit board comprising: a ground system;   a main power distribution system;   at least one sub-power plane disposed in prescribed spatial relationship with and physically separate from said main power distribution system, said sub-power plane being adapted to be positioned directly under and connected to at least one VLSI device having multiple power input leads;   a plurality of capacitors, said at least one sub-power plane being adapted to connect a first one of said plurality of capacitors to a first power input lead of said VLSI device and to connect at least a second one of said plurality of capacitors to a second power input lead of said VLSI device; and   a filter connecting said sub-power plane to said main power distribution system for supplying power to said sub-power plane.   
     
     
       10. A circuit board as defined in claim 9, wherein said filter comprises a capacitor, a ferrite bead and one of said plurality capacitors and is arranged in a π configuration. 
     
     
       11. A circuit board as defined in claim 10 wherein said ground system is a ground grid and wherein each of said capacitors has a terminal adapted to be connected to said ground grid.